One type of single point mooring system includes a transfer structure whose lower end is anchored to the sea floor to limit lateral drifting and rotation about a vertical axis. The upper end of the transfer structure is supported by a vessel by a joint that permits the vessel to rotate 360.degree. about the transfer structure. A transfer structure in the form of a column which extends along much of the height of the sea is desireable in many cases, because it permits a protected oil-carrying hard pipe to extend most of the height of the sea. One type of column has a lower end pivotally mounted by a 2-axis joint to a base at the sea floor. Such mounting is expensive. Another type, shown in U.S. Pat. No. 4,262,620 by Nooteboom uses a pair of chains instead of a column, and anchors the bottom of the chain-column with a largely horizontally-extending arm whose lower end is held in a pivot joint to the sea floor. Such mounting is also expensive, and produces uneven mooring forces in different drift directions. A column mooring system which enabled low cost mooring of the column while enabling the obtaining of uniform mooring forces in every direction of vessel drift and applying mooring forces that increase gently with progressively increasing vessel drift up to a large force during large vessel drifting, would be of great value.
The mooring and cargo-transfer structures which employ a floating vessel, have generally been useful for transferring cleaned hydrocarbons to a ship, but not generally for the production of hydrocarbons from undersea wells. In the production of hydrocarbons from undersea wells, the well effluent typically includes solid and fluid impurities including sand and water, as well as liquid and gas. Furthermore, a well typically produces at high pressures such as 6,000 psi. Reliable fluid swivels for permitting the vessel to drift 360.degree. about the transfer structure, have not been available to transfer fluids at such high pressures. Any sand or other particles present in the fluids would add to maintenance problems of any such swivel. Research has been conducted on the design of such fluid swivels, but it would appear that the cost and maintenance of the swivel could be prohibitive. Accordingly, undersea hydrocarbons are typically produced using large and expensive fixed platforms. An offshore terminal that permitted the production of hydrocarbons from undersea wells, by the use of floating vessels, would permit the production of undersea hydrocarbons at lower cost.